This invention relates to a synchrotron radiation SR source and more particularly to a SR source having beam absorbers suitable for realizing its compactness.
As discussed in "Proceeding of the 5th Symposium on Accelerator Science and Technology" in the high energy laboratory reports, 1984, pp. 234-236, conventional accelerators and large-scale SR sources are known wherein bending sections, each of which deflects the orbit of a charged particle beam for causing the synchrotron radiation to be taken out of the source, are not collectively disposed in a relatively short range of the beam duct or bending duct, but disposed with spaces between them where straight sections are disposed so that the bending sections are uniformly distributed as a whole in the beam duct or bending duct.
Accordingly, sources of gases discharged from the interior wall surface of the vacuum chamber under the irradiation of synchrotron radiation are substantially uniformly distributed along the orbit of the charged particle beam. Gases discharged from the bending sections under the irradiation of the synchrotron radiation can be evacuated not only by built-in pumps installed inside the charged particle bending section but also vacuum pumps installed in an adjacent straight section, thereby ensuring that the vacuum chamber can be maintained at high vacuum and a long lifetime of the charged particle beam can be maintained.
Conventionally, portions irradiated directly with the synchrotron radiation are made of a stainless steel material or an aluminum alloy material. When irradiated with the synchrotron radiation, the above material discharges a large amount of gases under the influence of the photo-excitation reaction.
Since the amount of discharged gases is very large amounting to 10 to 100 times the outgassing amount due to mere thermal discharge, a great number of vacuum pumps must be installed in order to maintain the interior of the vacuum chamber at a high vacuum.
Further, when the bending angle of the charged particle beam obtained by one tending section is designed to be large for the sake of realizing compactness of the SR source, the amount of gases discharged from one bending section is increased and a great number of vacuum pumps must be installed. However, because of a limited installation space, the number of pumps to be installed is limited, raising a problem that the interior of the vacuum chamber can not be maintained at high vacuum and the lifetime of the charged particle beam is shortened.
Moreover, in compact SR sources for industrial purposes, because cost reduction is desirable, the bending section for delivery of synchrotron radiation has to be laid concentratedly.
Taking a compact SR source comprised of two straight sections and two bending sections, for instance, it is necessary for one bending section to 180.degree. deflect the charged particle beam orbit and as a result, the amount of gases generated by each bending section under the irradiation of synchrotron radiation upon the interior wall surface of a portion of the vacuum chamber corresponding to one bending section is greatly increased, reaching about 10 times the amount of discharged gases generated by each bending section in the case of a large-scale SR source.
Accordingly, if the configuration of the vacuum chamber and the layout of vacuum pumps in the large-scale SR source are directly applied to the compact SR source without alternation, then there will arise a problem that pressure in the vacuum chamber rises and the lifetime of the charged particle beam is shortened.
A countermeasure for solving the above problems has been proposed wherein the shape of the bending section/vacuum chamber is made different from the conventional duct form of the bending section/vacuum chamber of the large-scale SR source so as to take the form of a sector or a semi-circle and in addition, vacuum pumps are installed near the outer circumferential wall of the bending section/vacuum chamber and SR guide ducts extend from the outer circumferential wall. With this proposal, the vacuum evacuation performance can be comparable or superior to that of the conventional large-scale SR source but disadvantageously the orbit of the charged particle beam tends to be unstable.
More particularly, the sector or semi-circular form of the bending section/vacuum chamber tends to adversely interfere with the orbit of the charged particle beam guided to the bending section, thereby inducing a high-frequency electric field (called a wake field) which makes the orbit of the charged particle beam unstable.